A ground fault is the accident grounding of a conductor. Conventional ground fault interrupters are employed in alternating-current (AC) circuits and are used to sense very small ground fault currents such as might flow through the body of a person standing on damp ground while touching a hot AC line wire. A ground fault interrupter for an AC circuit is positioned between the AC power source and load. The ground fault interrupter compares the AC supply current flowing from the power source to the load with the AC return current flowing from the load back to the power source. If the supply current differs from the return current by greater than a predetermined threshold, then the ground fault interrupter interrupts the flow of supply current from the power source to the load. To measure AC current differences between the supply current and the return current, the ground fault interrupter employs a toroidal magnetic circuit having a magnetic core and an equal number of supply and return coils of current-carrying wire wound about the magnetic core. The direction of current flow through these coils is designed such that the magnetic flux produced by the supply wire coils is cancelled by the magnetic flux produced by the return wire coils. A third winding is used to pick off any residual flux remaining due to differences in the supply and return currents.
The foregoing technique for measuring current differences between supply and return currents does not work for DC power supplies because the DC current generated by DC power supplies produces a constant magnetic flux which will not produce current in the third winding. To measure the constant magnetic flux, a Hall-effect device could be inserted into a gapped toroidal core. This solution, however, is inefficient and impractical because the number of wire coils wound about the gapped toroidal core would be excessive and factory tuning calibration might be necessary. A need therefore exists for an efficient and practical ground fault interrupter for a DC circuit.